He received the degrees BA Law cum laude, LLB cum laude, LLD and LLD honoris causa from the University of Pretoria. He also researched for lengthy periods in Germany (as an Alexander von Humboldt-fellow) and the USA and for shorter periods in the Netherlands and the United Kingdom..
He was professor in and the head of the Department of Legal History, Comparative Law and Jurisprudence in the University of Pretoria's Faculty of Law, as well as the founding director of the university's Centre for Human Rights. He is currently an honorary professor at the University of Pretoria and a member of the Centre’s board of trustees, the Law Faculty Council of the University of Pretoria and council of the Judicial Education Institute of South Africa
As an academic, Van der Westhuizen has taught widely in South Africa and abroad, including at Yale Law School and in Germany and Canada; presented numerous papers at national and international conferences; authored and edited several publications; and participated in numerous radio and television programmes in the USA, Germany, Canada, Japan and South Africa.
He was admitted as an advocate of the High Court of South Africa and was an associate member of the Pretoria Bar. He acted as counsel in many human-rights matters, and served as a consultant and in-house advocate for the Legal Resources Centre and on the governing body of Lawyers for Human Rights.
Constitutional Court judgments written by Justice Van der Westhuizen dealt with matters including constitutional amendments, provincial boundaries and powers, fair trial issues, equality, the development of African customary law, the right of access to adequate housing and other socio-economic rights, asset forfeiture and search and seizure procedures, the right to privacy and the contractual and delictual liability of private security companies, freedom of expression and land claims
He was appointed as an Inspecting Judge on the 01 April 2016.